Adjustable pipette

ABSTRACT

An adjustable pipette includes an elongated body defining a barrel portion and a plunger cavity. A plunger unit is mounted for reciprocation in the body and includes a plunger portion extending from the barrel portion into the plunger cavity, and further includes an operating portion extending rearwardly out of the body. The plunger unit includes a stop disposed within the barrel portion, and an adjustable rear stop for varying the volume of a pipetted sample is carried on a threaded shaft received in a nut fixed in the body. An indicator including rotatable indicia-carrying rings is coupled to the shaft for providing an indication of the volume setting of the pipette. A forward stop for movement of the plunger unit is provided by an overtravel member engaged by the plunger stop member. Travel of the plunger unit past the forward position compresses an overtravel spring, and a seal assembly is compressed by the overtravel spring into sealing engagement with the plunger portion of the plunger unit. A sleeve carried within the threaded shaft around the operating portion of the plunger unit is adjustable relative to the shaft in order to align the indicator relative to the shaft. A lock arrangement is provided for locking the shaft in position to prevent inadvertent misadjustment of the volume.

United States Patent 1191 1111 3,827,305 Gilson'et al. Aug. 6, 1974ADJUSTABLE PIPETTE [57] ABSTRACT Inventorsi Warren 4801 sheboygan Anadjustable pipette includes an elongated body de- R0136" Gllson, 4Franklin fining a barrel portion and a plunger cavity. A plunger both ofMadlson, 53705 unit is mounted for reciprocation in the body and in-[22] Filed: Oct 24 1972 cludes a plunger portion extending from thebarrel portion into the plunger cavity, and further includes PP 300,215an operating portion extending rearwardly out of the body. The plungerunit includes a stop disposed within 52 11.s.c1. 73/425.6, 222/43 the hP and adjustable Stop for [51 1m. 01. c0111 1/14 vary'hg the vhhhe aplpehedl Sample carried on [58] Field of Search 73/425.6, 425.4 P; athreaded shelft received in a hut fixed in the body- 128/234 218 F;141/27; 222/340, 341 43; An indicator including rotatableindicia-carrying rings 277/102 187 is coupled to the shaft for providingan indication of the volume setting of the pipette. A forward stop for[56] References Cited movelment of the plungierbunirt is provided by anogertrave mem er enga e yt e unger sto mem er. UNITED STATES PATENTSTravel of the plung r unit past the forwafd position 2,530,909 ll/l950R1ggs 222/309 X compresses an overtrave] Spring, and a Sea] assembly a i'figzi 43 2 is compressed by the overtravel spring into sealing en-3'49s135 3 1970 seitz et al. If 31173/42515 gagemem the P i Portia"ofthe plunger 3I6I3I952 10/1971 Gilmont 73/425.6 x A Sleeve camed thethreaded Shaft around the R27,637 5/1973 Roach 73/425.6 OperatingPortion Of-Ihe Plunger unit is adjustable Primary ExaminerRichard C.Queisser Assistant Examiner-Daniel M. Yasich Attorney, Agent, orFirmMason, Kolehmainen, Rathburn & Wyss ative to the shaft in order toalign the indicator relative to the shaft. A lock arrangement isprovided for locking the shaft in position to prevent inadvertentmisadjustment of the volume.

11 Claims, 4 Drawing Figures ADJUSTABLE PIPETTE The present inventionrelates to improvements in adjustable pipettes.

Traditional pipettes have not been able to meet modern requirements inthat they are not designed rapidly to transfer liquid samples havingprecise volumes. As a result many attempts have been made to provideimproved pipetting devices of various constructions which canconveniently be used to transfer samples rapidly and accurately.Examples of devices of this type can be found in U.S. Pat. No. 3,244,009Tietje et al. and U.S. Pat. No. 3,506,164 Weichselbaum et al.

Although such devices provide samples of precise quantity, they sufferfrom several disadvantages. A primary disadvantage is that such devicesare not capable of providing samples of continuously variable size. Atthe most, presently available devices are adjustable by the user betweentwo or a few discrete volumes. In addition, no clear indication isprovided of the volume setting, and this can result in errors or atleast inconvenience in use.

Precise adjustment together with accurate volume indications have beenavailable in the past with buretting devices. However, attempts madeheretofore to adapt these features to pipettes have not been sucessful.For example, U.S. Pat. No. 3,013,435 Rodrigues, Jr. discloses a burettedescribed as capable of being used as a pipette. Although this devicecan be continuously adjusted, the adjustment mechanism is extremelycomplex and awkward to use. Moreover, although this device provides anindication useful for buretting, the indication does not correspondconsistently with the actual volume setting when the device is used forpipetting. Consequently, the Rodrigues, in device not only would bedifficult or impossible to manufacture but also falls far short ofsolving the problems noted above with heretofore proposed pipettingdevices.

Among the important objects of the present invention are to provideimprovements in adjustable pipettes; to provide a pipette having asimplified and improved arrangement for continuous yet preciseadjustments in volume; to provide a pipette having a reliable,consistent, and accurate volume indicator; to provide a pipette whereinthe indicator can be aligned relative to the volume adjustment to assureaccuracy; to provide an improved seal assembly for pipettes; to providea pipette including structure for locking the volume adjustment inposition; and to provide an improved adjustable pipette of sturdy andcompact construction capable of economical manufacture.

In brief, an adjustable pipette constructed in accordance with thepresent invention includes an elongated body defining a barrel portionand defining a plunger cavity adjacent the forward end of the body. Aplunger unit is mounted for reciprocation in the body and includes aplunger portion extending from the barrel portion of the body into theplunger cavity, together with an operating portion extending rearwardlyout of the body. A seal assembly is carried by the body and sealsagainst the plunger portion. The plunger unit includes a stop engagingmember mounted on the plunger unit and disposed within the barrelportion of the body. A forward stop in the body is engaged by the stopengaging member upon forward movement of the plunger unit to a forwardstop position.

In order to adjust the sample volume, a hollow threaded shaft engages anut fixed in the body and includes a forward end disposed within thebarrel portion of the body and a rear end extending rearwardly out ofthe body. A rear stop is associated with the forward end of the threadedshaft and is engaged by the plunger unit stop engaging means uponrearward movement of the plunger unit to a rear position. An indicatorarrangement mounted with the body is keyed to the shaft for providing anindication in response to rotation of the shaft indicative of theposition of the rear stop and thus of the sample size.

The present invention together with the above and other objects andadvantages may be better understood with reference to the embodiment ofthe invention illustrated in the accompanying drawing, wherein:

FIG. 1 is a partly sectional side view of an adjustable pipetteembodying the features of the present invention;

FIG. 2 is an enlarged, fragmentary sectional view of the adjustablepipette of FIG. 1 taken along the line 2-2 of FIG. 1;

FIG. 3 is an end view of the adjustable pipette partly taken in sectionalong the line 3-3 of FIG. 2; and

FIG. 4 is an exploded, fragmentary perspective view of parts of the sealassembly of the adjustable pipette.

Having reference now to the drawing, there is illustrated an adjustablepipette generally designated by the reference numeral 10 and constructedin accordance with the principles of the present invention. The pipette10 includes a body designated as a whole by the reference numeral 12within which there is reciprocably supported a plunger unit 14. Inaccordance with one feature of the invention, there is provided a novelseal assembly generally designated as 16 providing an air tight sealbetween the body 12 and the plunger unit 14. A volume adjustmentassembly generally designated as 18 functions in accordance with anotherfeature of the invention to permit reliable and accurate smallincrementvolume adjustments, and volume settings are clearly indicated inaccurate fashion by a volume indicator assembly generally designated as20.

Proceeding now to a more detailed description of the structure of theadjustable pipette 10, the body 12 includes a main housing 22 togetherwith a nose member 24. The main housing 22 defines a generallycylindrical barrel portion 26 and an enlarged cavity 28 for enclosingthe volume indicator assembly 20. The rear end of the main housing 22 isclosed by a cap 30. The cap may be attached in any suitable manner, andpreferably is snapped into place by a protuberance and notch arrangement32 (FIG. 2).

Mounting of the nose member 24 to the main housing 22 is accomplished bymeans of a nut 34 threaded onto the forward end of the main housing 22over the barrel portion 26. The nose member 24 includes an enlargedrearward end section 36 defining an overtravel chamber 38. A mountingrib 40 is held firmly against the forward end of the main housing 22 bya flange 42 of nut 34, and a spring retainer 44 is captured between therearward end of the nose member 24 and a shoulder 46 defined by acounterbore in the forward end of the barrel portion 26 of the mainhousing 22.

Outwardly of the enlarged end 36, the nose member 24 includes anelongated section 48 defining a plunger cavity 50. In order to avoidcontamination of samples from prior samples, preferably the forward endof the nose member 24 is provided with frictionally mounted, tapered tipmembers such as the member 52 illustrated in FIG. 1. An opening 54 inthe nose member 24 provides communication between the interior of thetip 52 and the plunger cavity 50. The interior of the tip 52 ispreferably large enough so that entire samples may be held thereinwithout contact with the nose member 24, and preferably the tip member52 is made of suitable non-wetting plastic material to facilitatecomplete discharge of samples.

Advantageously, the overall size and configuration of the adjustablepipette 10 are such as to facilitate its use. The slender configurationpermits insertion of the nose member 24 into narrow receptacles. Theadjustable pipette 10 can readily be held in the hand, and the mainhousing 22 is provided with a trigger guard-type finger handle 56 havinga generally C-shape or D-shape and substantially completely encirclingthe forefinger of a hand grasping the main housing 22. The arrangementis such that in use the adjustable pipette can be held suspended upon aforefinger by the finger handle 56 in position naturally to be graspedby the hand with the thumb in position for use over the rearward end ofthe pipette 10.

Samples are drawn into and discharged from the tip member 52 by means ofreciprocation of the plunger unit 14 within the body 12. The plungerunit 14 includes a circular cylindrical plunger portion 60 having aforward end extending into the plunger cavity 50 defined within the nosemember 24. Extending rearwardly from the plunger portion 60, andpreferably integral therewith, is an operating rod portion 62terminating well outside of the rearward end of the body 12. A thumbbutton 64 is attached to the outer end of the operating rod portion 62by a screw retainer 66.

In its rest position the plunger unit 14 is urged rearwardly by a spring67 (FIG. 2) against an adjustable rear stop, described hereinafter andcomprising part of the volume adjustment assembly 18. The spring 67 isheld in compression within the barrel portion 26 of the housing 22between the spring retainer 44 and a spring retainer and stop member 68.The member 68 is located over a shoulder 70 between the plunger portion60 and operating rod portion 62 of the plunger unit '14, and is pressfit to the plunger portion 60 or is otherwise permanently attached toform part of the plunger unit 14. Member 68 is provided with arearwardly facing stop surface 72 held against the adjustable rear stopin the rest position of the plunger.

In order to draw up a sample into the tip member 52, the thumb button 64is depressed to move the plunger unit 14 forward. This movement isaccommodated by compression of spring 67. Member 68 includes anelongated small diameter forward end portion 74 comprising a forwardstop surface. Forward movement of the plunger unit 14 results inmovement of stop portion 74 through a central opening 76 in springretainer 44 until it strikes a forward stop and overtravel member 78mounted in the overtravel chamber 38 against the forward side of springretainer 44. In this position, the plunger portion 60 of the plungerunit 14 is extended into the plunger cavity 50. Tip member 52 isimmersed in a sample material, and the thumb button 64 is released.Plunger unit 14 is moved rearwardly by the force stored in spring 67 toretract the plunger portion 60 in the plunger cavity 50, and suctionwithin the plunger cavity 50 communicated to the interior of the tipmember 52 draws a sample of predetermined size into the tip member 52.

in order to discharge the sample from tip member 52, the thumb button 64is once again depressed to move the plunger unit 14 in a forwarddirection. Upon the discharge stroke, the member 68 again movesforwardly until surface 74 contacts the forward stop and overtravelmember 78. This member is held in the position illustrated in FIG. 2 bymeans of an overtravel spring 80 held in compression between the forwardside of member 78 and the rearward side of a spring retainer andpressure member 82 disposed at the forward end of the overtravel chamber38. During a discharge stroke, the plunger unit 14 is moved through anovertravel movement wherein the forward stop portion 74 of the member 68drives the forward stop and overtravel member 78 against the force ofthe spring 80 into the overtravel chamber 38. As a result of thisovertravel movement the plunger portion 14 moves further into theovertravel chamber 50 during the discharge stroke than during the intakestroke. Thus, it is assured that the entire sample is fully dischargedfrom the tip 52. In order to provide a positive, easily detected stop atthe end of the intake movement, the spring 80 is preferablysubstantially stiffer than the spring 67.

In accordance with a feature of the invention, the seal assembly 16 ofnovel construction provides a reliable, airtight, low friction sealbetween the body 12 and the plunger unit 14. Moreover, seal assembly 16is spring loaded by over-travel spring 80 and thus spring 80 performs adual function.

More specifically, as shown in FIGS. 2 and 4, the seal assembly 16includes a toric seal member 86 disposed within a counterbore cavity 88in the forward end of the spring retainer and pressure member 82. Sealmember 86 includes a cylindrical collar portion 90 intimately encirclingthe plunger portion 60 and a flange section 92 engaged by the member 82.A resilient O- ring 94 is held around the collar 90 between the flange92 of the seal member and the forward wall of the overtravel cavity 38.The size of the O-ring 94 relative to the size of the toric seal member86 and cavity 88 is such that the O-ring 94 is somewhat compressed bythe force of the spring 80. The toric seal member is of somewhat softand flexible plastic material, and the cylindrical collar portion 90 ofthe seal member 88 is resiliently forced by the spring loaded O-ring 94inwardly into sealing relation with the plunger portion 60. The use of asmooth metal plunger unit 14 and of a flexible plastic seal member 86assures not only a good seal, but also very low sliding friction duringreciprocation of the plunger unit 14.

An important advantage of the adjustable pipette 10 of the presentinvention is that it can be easily adjusted in a continuously variablemanner and with great preci sion and accuracy. The volume adjustmentassembly 18 provided in accordance with the present invention includes ahollow threaded shaft having an inner end disposed within the barrelportion 26 of the housing 22 and having an outer end spaced outwardly ofthe cap 30. The shaft 100 is received in an internally threaded nutmember 102 fixed against rotational and longitudinal movement by meansof a locking pin 104. Nut 102 is received in the rearward end of thebarrel portion 26 and includes a flange 106 received in the forward endof the enlarged cavity 28. A retaining nut 108 is threaded onto theinnermost end of the shaft 100 and is then looked in place, as bydeformation. The purpose of the retaining nut 108 is to prevent theshaft 100 from being inadvertently withdrawn through the nut member 102.

Rotation of the shaft 100 is facilitated by the provision of a knurledhead or cap 110 threaded over the outer end of the shaft 100 and lockedinto position by means of a pair of screw retainers 112. Slidablyreceived over the operating rod portion 62 of the plunger unit 14 withinthe hollow threaded shaft 100 is an inner sleeve or shaft 114. Thepurpose of sleeve 114, described more fully hereinafter, is to permitadjustment of the volume indicator assembly 20 in relation to the volumeadjustment assembly 18. The forward end of the sleeve 114, designated bythe reference numeral 116, extends slightly beyond the forward end ofthe threaded shaft 100 and forms the forwardmost part of the volumeadjustment assembly 18. Thus the forward end 116 of sleeve 114 isengaged by the stop surface 72 of the member 68 and provides therearward stop for motion of the plunger unit 14.

Adjustment of the sample volume of pipette is accomplished by rotationof the head 110 resulting in reciprocation of the threaded shaft 100 andof the sleeve 114 due to threaded engagement with the nut member 102.This reciprocation moves the rearward stop 116 toward and away from theforward stop 78 thus adjusting the stroke of the plunger member 14 andthe volume of a sample.

The volume adjustment assembly 18 also includes provision for lockingthe unit in position for any desired sample size. For this purpose thereis provided a locking ring 118 threaded upon the shaft 100 to the rearof the cap 30 of the main housing 22. Once the shaft has been rotated toposition it in the desired longitudinal position, the locking ring 118is threaded inwardly firmly against the cap 30 thus locking the shaft100 against further rotation.

In order to prevent movement of the locking ring 118 more than a slightdistance away from the cap 30, there is provided a retaining pin 120extending outwardly from cap 30 into a circular groove 122 in theunderside of locking ring 118. If an attempt is made to unscrew thelocking ring 118 away from the cap 30, the retaining pin 120 is engagedas illustrated in FIGS. 2 and 3 by a stop pin 124 extending across thegroove 122 in a radial direction. The arrangement is such that rotationof the locking ring 118 in the opposite direction away from the positionillustrated in FIG. 3 will result in tightening of the ring 118 againstthe cap 30 in less than a full revolution.

In accordance with another feature of the invention, the volumeindicator assembly provides an exact and easily perceived readout of thevolume setting of the volume adjustment assembly 18. Arranged in stackedrelationship within the enlarged cavity 28 between cap and flange 106are a series of indicator rings 130, 132 and 134, each encircling theadjustment shaft 100. In the illustrated embodiment of the invention therings 130, 132 and 134 express the volume setting in units, tens andhundreds, respectively, of microliters. It will be understood that feweror more rings graduated in any desired units may be provided.

In order to couple the indicator assembly 20 with the volume adjustmentassembly 18, the threaded shaft 100 includes a groove 136 extendingthroughout its length.

The units ring includes an inwardly extending projection 138 received inthe groove 136 to the end that the ring 130 rotates with the shaft 100but permits the shaft 100 to move longitudinally relative to the ring.The tens ring 132 and the hundreds ring 134 are free for relativerotation independent of the threaded shaft 100.

Movement of the rings 130, 132 and 134 relative to one another isaccomplished in a conventional manner. As a result of one fullrevolution of the units ring 130, a single pair of gear teeth 140 engagea spur gear 142 mounted on a shaft 144 extending axially through thelength of the enlarged cavity 28 of the housing 22. The spur gear 142engages continuous gears 146 on the tens ring 132 in order to move it asingle increment of a tenth of a revolution. Similarly, each fullrotation of the tens ring 132 results in incremental movement of thehundreds ring 134 by virtue of a single pair of gear teeth 148, anadditional spur gear 150 and continuous gear teeth 152 on the hundredsring 134.

As appears in FIG. 1, there is provided a transparent window 154 forviewing of the indicator rings 130, 132 and 134. Preferably the entiremain housing 22 is formed of clear plastic with the window area 154being polished for clear visibility. Any other window structure may beused if desired. Each of the rings 130, 132 and 134 is provided withnumerical indicia, and if desired the units ring 130 may be providedwith fractional graduations registering with an index line 156 on thewindow 154.

In order to assure accuracy of the indication provided by the assembly20, there is provided a zero adjustment feature. In the initialfabrication and assembly of the pipette 10, the rings 130, 132 and 134are mounted so that a zero indication appears when the shaft 100 isthreaded inwardly approximately until the stop portion 74 is in contactwith the forward stop 78 in its rearward position. In order precisely toadjust the zero position, the funtion of the slleeve 114 comes intoplay. The head 110 is rotated until the indicator assembly 20 providesprecisely at zero indication. At this point the screw retainers 112 areloosened and the head 110 can be rotated while the shaft 100 remainsstationary. During such rotation the reading of the indicator 20 is notchanged. However, rotation of the head 110 advances or retracts thesleeve 114 relative to shaft 100 to precisely locate the plungerassembly 14 at the zero volume position. When this position has beenreached, the screw retainers 112 are tightened once agin and the zeroadjustment is completed.

The operation of the adjustable pipette 10 will be apparent to thoseskilled in the art in light of the above detailed description. In use,the pipette is first adjusted through manipulation of the volumeadjustment assembly 18 to the desired volume. This is accomplished byrotation of the head 110 to cause reciprocation of the shaft 100 thussetting the position of the rear stop 116. When the desired volume asappearing at the window 154 has been attained, the locking ring 118 isrotated firmly against the cap 30 in order to prevent furtherinadvertent rotation of the shaft 100.

To draw up a sample, the thumb button 64 is depressed to drive theplunger portion 60 of the plunger unit 14 forwardly into the plungercavity 50 through a precisely predetermined distance until the stopportion 74 strikes the forward stop 78. Inadvertent overtravel isavoided due to the stiffness of the overtravel spring 80. At this pointthe tip member 52 is immersed in a sample material and the thumb button64 is retracted to draw up the sample.

The sample is discharged by once again depressing the thumb button 64 tomove the plunger portion 60 into the plunger cavity 50. During thedischarge stroke the plunger unit 14 is moved into the overtravel rangeby forcing the forward stop and overtravel member 78 forwardly into theovertravel chamber 38 against the force of the stiff spring 80. Thisovertravel movement assures complete discharge of the sample. When thethumb button 64 is released, the plunger unit 14 re turns to its restposition with the surface 72 against the rear stop 116.

Although the invention has been described with reference to details ofthe illustrated embodiment, it should be understood that such details donot limit the invention which is defined in the appended claims.

What is claimed and desired to be secured by Letters Patent of theUnited States is:

1. An adjustable pipette comprising in combination;

an elongated body defining a barrel portion and defining a plungercavity adjacent the forward end of said body;

a plunger unit mounted for reciprocation in said body and including aplunger portion extending from said barre-l portion into said plungercavity and an operating portion extending rearwardly out of said body;

first biasing means in said barrel portion urging said plunger unit in arearward direction;

said body defining an overtravel chamber disposed between said barrelportion and said plunger cavy;

a seal assembly resiliently carried in the overtravel chamber in saidbody in sealing relation with said plunger portion;

said plunger unit including a stop engaging member permanently mountedthereon and disposed within said barrel portion;

an overtravel member mounted for movement in said overtravel chamber andhaving a forward stop surface engageable by said stop engaging memberupon forward reciprocation of said plunger unit to a forward stopposition;

second biasing means in said overtravel chamber for urging saidovertravel member against the rearward end of said overtravel chamber;

an internally threaded nut fixed in said body against rotational andaxial movement;

a hollow shaft threaded through said nut for rotational and reciprocalmovement and including a forward end disposed within said barrel portionand a rear end extending rearwardly out of said body;

said operating portion of said plunger unit extending through saidshaft;

rear stop means reciprocal with the forward end of said shaft uponrotation of said shaft in said nut and engageable by said stop engagingmember upon rearward reciprocation of said plunger unit to a rear stopposition; and

indicator means in said body coupled to said shaft for providing anindication in response to rotation of said shaft indicative of theposition of said rear stop means.

2. The pipette of claim 1, said second biasing means being substantiallystiffer than said first biasing means to provide a positive feel of theforward stop position.

3. The pipette of claim 1, said overtravel chamber having a forwardwall, said seal assembly being compressed between said second biasingmeans and said forward wall around said plunger portion.

4. The pipette of claim 3, said seal assembly including a retainermember pressed by said second biasing means toward said forward wall andhaving an axial central opening receiving said plunger portion, acounterbore cavity in the forward side of said retainer membercooperating with said forward wall to define an annular chambersurrounding said plunger portion, and compressible sea] means in saidannular chamber.

5. The pipette of claim 4, said compressible sealing means including aflexible cylindrical collar surrounding said plunger portion, and anO-ring surrounding said collar, said O-ring being sized to deform whencompressed by said second biasing means to force said collar inwardlyagainst said plunger portion.

6. The pipette of claim 1, a locking member threaded upon said shaft tothe rear of said body, said locking member being threadable to a lockingposition against the rear of said body selectively to lock said shaftagainst rotation.

7. The pipette of claim 6 including retaining means on said body and onsaid locking member for preventing rotation of said locking memberthrough a complete revolution from the locking position.

8. The pipette of claim 1, said indicator means including a plurality ofindicator rings encircling said threaded shaft within said body, indiciaon said indicator rings, a window means in said body for viewing of saidindicia, one said ring being coupled to said threaded shaft for rotationtherewith while permitting reciprocal movement of said shaft throughsaid rings, and drive means coupled between said rings for controllingthe relative movement thereof in response to rotation of said shaft.

9. The pipette of claim 8, said body being formed at least in part ofclear plastic, said window means comprising a region of said plastichaving a polished surface.

10. The pipette of claim 8, the position of said rear stop means beingadjustable relative to said shaft for adjustment of said rings relativeto the rear stop position of said plunger unit.

11. The pipette of claim 10, a cylindrical sleeve slightly longer thansaid shaft slidably received within said shaft around said operatingportion of the plunger unit, a head member threaded on the rearward endof said shaft and engaging the rearward end of said sleeve, the forwardend of said sleeve forming said rear stop means, and said head beingrotatable on said shaft to reciprocate said sleeve relative to saidshaft.

1. An adjustable pipette comprising in combination; an elongated bodydefining a barrel portion and defining a plunger cavity adjacent theforward end of said body; a plunger unit mounted for reciprocation insaid body and including a plunger portion extending from said barrelportion into said plunger cavity and an operating portion extendingrearwardly out of said body; first biasing means in said barrel portionurging said plunger unit in a rearward direction; said body defining anovertravel chamber disposed between said barrel portion and said plungercavity; a seal assembly resiliently carried in the overtravel chamber insaid body in sealing relation with said plunger portion; said plungerunit including a stop engaging member permanently mounted thereon anddisposed within said barrel portion; an overtravel member mounted formovement in said overtravel chamber and having a forward stop surfaceengageable by said stop engaging member upon forward reciprocation ofsaid plunger unit to a forward stop position; second biasing means insaid overtravel chamber for urging said overtravel member against therearward end of said overtravel chamber; an internally threaded nutfixed in said body against rotational and axial movement; a hollow shaftthreaded through said nut for rotational and reciprocal movement andincluding a forward end disposed within said barrel portion and a rearend extending rearwardly out of said body; said operating portion ofsaid plunger unit extending through said shaft; rear stop meansreciprocal with the forward end of said shaft upon rotation of saidshaft in said nut and engageable by said stop engaging member uponrearward reciprocation of said plunger unit to a rear stop position; andindicator means in said body coupled to said shaft for providing anindication in response to rotation of said shaft indicative of theposition of said rear stop means.
 2. The pipette of claim 1, said secondbiasing means being substantially stiffer than said first biasing meansto provide a positive feel of the forward stop position.
 3. The pipetteof claim 1, said overtravel chamber having a forward wall, said sealassembly being compressed between said second biasing means and saidforward wall around said plunger portion.
 4. The pipette of claim 3,said seal assembly including a retainer member pressed by said secondbiasing means toward said forward wall and having an axial centralopening receiving said plunger portion, a counterbore cavity in theforward side of said retainer member cooperating with said forward wallto define an annular chamber surrounding said plunger portion, andcompressible seal means in said annular chamber.
 5. The pipette of claim4, said compressible sealing means including a flexible cylindricalcollar surrounding said plunger portion, and an O-ring surrounding saidcollar, said O-ring being sized to deform when compressed by said secondbiasing means to force said collar inwardly against said plungerportion.
 6. The pipette of claim 1, a locking member threaded upon saidshaft to the rear of said body, said locking member being threadable toa locking position against the rear of said body selectively to locksaid shaft against rotation.
 7. The pipette of claim 6 includingretaining means on said body and on said locking member for preventingrotation of said locking member through a complete revolution from thelocking position.
 8. The pipette of claim 1, said indicator meansincluding a plurality of indicator rings encircling said threaded shaftwithin said body, indicia on said indicator rings, a window means insaid body for viewing of said indicia, one said ring being coupled tosaid threaded shaft for rotation therewith while permitting reciprocalmovement of said shaft through said rings, and drive means coupledbetween said rings for controlling the relative movement thereof inresponse to rotation of said shaft.
 9. The pipette of claim 8, said bodybeing formed at least in part of clear plastic, said window meanscomprising a region of said plastic having a polished surface.
 10. Thepipette of claim 8, the position of said rear stoP means beingadjustable relative to said shaft for adjustment of said rings relativeto the rear stop position of said plunger unit.
 11. The pipette of claim10, a cylindrical sleeve slightly longer than said shaft slidablyreceived within said shaft around said operating portion of the plungerunit, a head member threaded on the rearward end of said shaft andengaging the rearward end of said sleeve, the forward end of said sleeveforming said rear stop means, and said head being rotatable on saidshaft to reciprocate said sleeve relative to said shaft.